Finally I learnt how to use
Amarok, the awesome Linux/KDE player, to generate guided random playlists, something I was looking for for a long time. All that was needed was to tag songs I do not want to listen to on "casual" random mode (progressive rock epics, music that needs full immersion to be enjoyed properly, catharsis-inducing songs, etc.). Doing so is a quite comfortable procedure with the great Collection interface of Amarok. Then, I just had to define a "Smart Playlist" by criteria based on those multiple tags and incorporate it in a custom "Dynamic Playlist" mode, which continuously adds random songs form the Smart Playlist to the queue. The overall result is really great, so expect my playcount on last.fm to grow by a sizable amount just because of that...

(Screenshot attached for anyone curious)